1. Technical Field of the Invention
The present invention relates to a developing roller, a developing apparatus, and an image forming apparatus, and particularly to a developing roller, a developing apparatus, and an image forming apparatus for carrying toner to a photoconductive unit and developing a toner image on the photoconductive unit.
2. Related Art
Conventionally, an image forming apparatus such as a copy machine, printer or multi-functional peripheral (MFP) employs an electrophotographic system in which an electrostatic latent image on a photoconductive drum is developed by a developing apparatus having a developing roller.
Of the surface area of the developing roller, an area facing the photoconductive drum is called a developing area. The developing apparatus causes a stirred developer (in the case of a two-component developer, it contains toner and magnetic particles) to adhere to the surface of the developing roller, and carries it to the developing area. The toner is electrically charged by being stirred and is attracted from the developing roller to the photoconductive drum because of the potential difference between the developing roller and the photoconductive drum in the developing area. Thus, the electrostatic latent image on the photoconductive drum is developed as a toner image.
To cause the developer to adhere to the surface of the developing roller and carry the developer to the developing area, the surface of the developing roller must have friction resistance. Therefore, surface treatment to form small recesses and protrusions is performed on the surface of the developing roller, instead of forming a perfectly smooth surface.
As conventionally known surface treatment for the developing roller, sandblasting and knurling may be employed.
A surface processed by sandblasting is a granulated rough surface and has high friction resistance. However, as it is used for a long period, the surface is ground by the developer and its surface roughness is gradually lowered. Consequently, friction resistance is lowered and the developer cannot be carried stably.
On the other hand, knurling is processing to form multiple grooves parallel to the axis of the developing roller, on the surface of the developing roller, as disclosed in U.S. Pat. No. 6,925,277 and so on. Each groove has a length that is approximately the same as the length of the developing roller in the axial direction. The multiple grooves formed on the surface generate high friction resistance to the developer. Moreover, since deeper recesses can be formed than in sandblasting, friction resistance is not significantly lowered even if the developing roller is used for a long period.
However, when development is carried out with the developing roller processed by knurling, there is a problem that an unwanted stripe pattern corresponding to the grooves are generated on the developed toner image. This unwanted stripe pattern is particularly conspicuous in a broad, solid area such as a blue sky background.
With the developing roller processed by sandblasting, such an unwanted stripe pattern is not generated, but there is a problem of performance deterioration due to change with time.